Term marginal returns Definition: The change in the quantity of total product resulting from a unit change in a variable input, holding all other inputs fixed. You might recognize this as the definition of marginal product. It is. Marginal returns is an older and more generic term for marginal product. While marginal product has largely replaced marginal returns in most discussions of short-run production, the phrase does persist in a few terms like the law of diminishing marginal returns. When you come upon the phrase marginal returns, more often than not, it's probably referring to marginal product.